The present invention relates to a resist pattern forming method and a semiconductor device fabrication method, more specifically a resist pattern forming method for forming a resist pattern having a downsized opening beyond a resolution of a photoresist and a semiconductor device fabrication method using said the resist pattern forming method.
In field effect transistors using compound semiconductor materials, evaporation/lift-off method is widely used in forming the gate electrodes and ohmic electrodes so as to decrease damages to the semiconductor surfaces (refer to, e.g., Reference 1 (Japanese published unexamined patent application No. Hei 07-153666), Reference 2 (Japanese published unexamined patent application No. Hei 08-115923), and Reference 3 (Japanese published unexamined patent application No. Hei 11-307549)). In the process of forming the electrodes by lift-off method, photoresist films having openings the end of which is reverse-tapered or formed in eave-shape is formed.
As one means for forming a photoresist film having such configuration is known a method using 2 kinds of photoresist materials of different sensitivities. In this method, the photoresist film of a lower sensitivity is formed on the photoresist film of a higher sensitivity, and these photoresist films are concurrently exposed and developed. The opening width is larger in the photoresist film of the higher sensitivity than in the photoresist film of the lower sensitivity, whereby a photoresist film having an eave-shaped end can be formed. As another means for forming a photoresist film having such configuration is proposed a method using a UV sensitive resist with a UV absorbing dye added.
FIGS. 13A-13D and 14A-14C are sectional views of the semiconductor device in the steps of the conventional semiconductor device fabrication method using the evaporation/lift-off method and the two-layer resist process, which show the method.
A silicon nitride film, for example, is deposited on a compound semiconductor substrate 100 by, e.g., CVD method to form an insulating film 102 of the silicon nitride film (FIG. 13A).
Next, a photoresist film 104 exposing a region for the gate electrode to be formed in is formed on the insulating film 102 by photolithography (FIG. 13B).
Then, with the photoresist film 104 as the mask, the insulating film 102 is dry-etched to form an opening 106 in the insulating film 102 down to the compound semiconductor substrate 100.
Next, the photoresist film 104 is removed by, e.g., ashing method (FIG. 13C).
Then, on the insulating film 102 with the opening 106 formed in, a photoresist film 108, and a photoresist film 110 which is different from the photoresist film 108 in the optical characteristics and the etching characteristics.
Next, the photoresist film 108 is patterned by photolithography to form an opening 112 in the photoresist film 110 in a region containing the region for the opening 106 formed in.
Then, with the photoresist film 110 as the mask, the photoresist film 108 is isotropically etched by, e.g., wet etching to form in the photoresist film 108 an opening 114 which is wider than the opening 112 (FIG. 13D).
Next, a metal film 116 to be the gate electrode is deposited by, e.g., evaporation method (FIG. 14A).
Then, the photoresist films 108, 110 are removed with, e.g., a resist releasing agent. Concurrently therewith the metal film 116 on the photoresist film 110 is removed (lift-off) together with the photoresist films 108, 110. Thus, the gate electrode 118 of the metal film 116 connected to the compound semiconductor substrate 100 through the opening 106 can be formed (FIG. 14B).
On the other hand, as semiconductor devices are more downsized, the wavelength of the exposure light used in the photolithography is made shorter. However, the making the wavelength of the exposure light shorter needs large expenses and much time for the studies and investments of photoresist materials having resolutions corresponding to shorter wavelengths, exposure systems, etc. The electron beam lithography has found it difficult to ensure the same throughput as the photolithography. Techniques of forming more downsized patterns than resolutions of the photoresists are being studied, and techniques of lasting the exposure light wavelengths are being studied.
In such background, as a technique of forming a photoresist film having a downsized pattern beyond the resolution of the photoresist film, a prescribed auxiliary agent is reacted with the patterned photoresist film to swell the resist pattern to thereby more downsize the pattern formed by the photolithography is developed (refer to, e.g., Reference 4 (Japanese published unexamined patent application No. Hei 10-073927) and Reference 5 (“Advanced Micro-Lithography Process for I-line Lithography”, T, Ishibashi et al., Jpn. J. Appl. Phys. vol. 40 (2001) p. 7156)).